This invention relates to a process and catalyst for reducing the aromatics and olefins content of hydrocarbon distillate products. More particularly, this process relates to an improved catalytic hydrogenation process and catalyst wherein the catalyst comprises platinum and palladium incorporated onto a support comprising borosilicate.
For the purpose of the present invention, the term "hydrogenation" is intended to be generic to the terms "hydrotreating" and "hydroprocessing," and involves the conversion of hydrocarbons at operating conditions selected to effect a chemical consumption of hydrogen. Included within the processes intended to be encompassed by the term "hydrogenation" are aromatic hydrogenation, dearomatization, ring-opening, hydrorefining (for nitrogen removal and olefin saturation), and desulfurization (often included in hydrorefining). These processes are all hydrogen-consuming and generally exothermic in nature. For the purpose of the present invention, distillate hydrogenation does not include distillate hydrocracking which is defined as a process wherein at least 15% by weight of the distillate feedstock boiling above 430.degree. F. at atmospheric pressure is converted to products boiling below 430.degree. F.
Petroleum refiners are now facing the scenario of providing distillate fuels, boiling in the range of from about 150.degree. F. to about 700.degree. F., with substantially reduced sulfur and aromatics contents. Sulfur removal is relatively well defined, and at constant pressure and adequate hydrogen supply, is generally a function of catalyst and temperature.
Aromatics removal presents a substantially more difficult challenge. Aromatics removal is generally a function of pressure, temperature, catalyst, and the interaction of these variables on the chemistry and thermodynamic equilibria of the dearomatization reaction. The dearomatization process is further complicated by the wide variances in the aromatics content of the various distillate component streams comprising the hydrogenation process feedstock, the dynamic nature of the flowrates of the various distillate component streams, and the particular mix of mono-aromatics and polycyclic aromatics comprising the distillate component streams.
The criteria for measuring aromatics compliance can pose additional obstacles to aromatics removal processes. The test for measuring aromatics compliance can be, in some regions, the FIA aromatics test (ASTM D1319), which classifies mono-aromatics and polycyclic aromatics equally as "aromatics." Hydrogenation to mono-aromatics is substantially less difficult than saturation of the final ring due to the resonance stabilization of the mono-aromatic ring. Due to these compliance requirements, hydrogenation to mono-aromatics may be inadequate. Dearomatization objectives may not be met until a sufficient amount of the polycyclic aromatics and mono-aromatics are fully converted to saturated hydrocarbons.
While dearomatization can require a considerable capital investment on the part of most refiners, dearomatization can provide ancillary benefits. Distillate aromatics content is inextricably related to cetane number, the accepted measure of diesel fuel quality. The cetane number is highly dependent on the paraffinicity of molecular structures, whether they are straight-chain or alkyl attachments to rings. A distillate stream which comprises mostly aromatic rings with few or no alkyl-side chains generally is of lower cetane quality while a highly paraffinic stream is generally of higher cetane quality.
Dearomatization of refinery distillate streams can increase the volume yield of distillate products. Aromatic distillate components are generally lower in API gravity than their similarly boiling paraffinic counterparts. Saturation of aromatic rings can convert these lower API gravity aromatic components to higher API gravity saturated components and expand the volume yield of distillate product thereby compensating partially for the cost of hydrogen and facilities.
Dearomatization of refinery distillate streams can also provide increased desulfurization and denitrogenation beyond ordinary levels attendant to conventional distillate hydrogenation processes. Processes for the dearomatization of refinery distillate streams can require the construction of a new dearomatization facility, the addition of a second-stage dearomatization step to an existing distillate hydrogenation facility, or other processing options upstream of distillate hydrogenation or at the hydrogenation facility proper. These dearomatization steps can further reduce the nitrogen and sulfur concentrations of the distillate component and product streams, thus reducing desulfurization and denitrogenation catalyst and temperature requirements in existing distillate hydrogenation facilities designed primarily for hydrorefining. Reduced distillate sulfur and nitrogen concentrations can additionally increase the value of these streams for use as blending stocks to sulfur-constrained liquid fuel systems and as fluid catalytic cracking unit (FCC) feed.
While distillate dearomatization can provide cetane number improvement, volume expansion, and additional desulfurization and denitrogenation, the process has seldom been attractive in view of the large capital costs and the fact that many refiners have not reached distillate cetane limitations. Now that legislation exists and further legislation is being considered to mandate substantial reductions in distillate aromatics content, the demand for distillate dearomatization processes is now being largely determined by the incentive to remain in the business of selling distillates.
Hydrogenation processes and catalysts for the treatment of distillate streams has been the subject of several patents. U.S. Pat. Nos. 3,736,252, 3,773,654, 3,969,222, 4,014,783, 4,070,272, 4,202,753, 4,610,779, and 4,960,505 are all directed towards processes for hydrogenating and dearomatizing distillate fuels.
The use of borosilicate in catalyst supports for distillate dearomatization is particularly rare. The molecular sieves including borosilicate and the zeolites in general, have not been commonly used in hydrogenation processes because the silica content, in combination with common commercial hydrogenation metals, such as nickel, molybdenum, and cobalt, can provide lower desulfurization activity, have a tendency to promote undesired cracking reactions, and can be prone to early deactivation. For this reason, catalyst support components comprising borosilicate have been more typically found in processes such as catalytic cracking, hydrocracking, dewaxing of lubricating oils, naphtha dehydrogenation, and the isomerization of alkyl aromatics.
U.S. Pat. No. 4,560,469 to Hopkins et al. discloses such a process for catalytically dewaxing lubricating oils utilizing a catalyst comprising nickel on a borosilicate molecular sieve. The dewaxing process is directed at converting high pour point paraffins present in lubricating oils to lower pour point components to improve automobile cold start performance.
U.S. Pat. No. 4,433,190 to Sikkenga et al. discloses a process for dehydrogenating normal butane into a mixture of isobutylene, isobutane, and normal butene using a catalyst comprising an AMS-1B crystalline borosilicate sieve and containing an ion or molecule of a noble metal. The dehydrogenation process is directed at dehydrogenating normal butane, an inexpensive refinery by-product, into a feedstock for polymerization processes.
U.S. Pat. No. 4,654,456 to Nimry discloses a process for isomerizing xylene utilizing a catalyst comprising a HAMS-1B crystalline borosilicate molecular sieve impregnated with a phosphorous compound.
The use of metal mixtures on a catalyst support has also been the subject of research. (See P. N. Rylander, Catalytic Hydrogenation over Platinum Metals, Academic Press, New York 1967.) Rylander teaches that two platinum metal catalysts, when used together, can give better rates or better yields than either catalyst individually. However, except for certain selected examples, there seems to be no way of predicting when mixtures of catalyst will prove advantageous. A useful guide as to the probable effectiveness of coprecipitated metal catalysts, is the performance of a mechanical mixture of the two metals. (See Rylander, at pages 9-11.)
U.S. Pat. No. 3,943,053 to Kovach et al. discloses a hydrogenation process using a catalyst comprising a particular mixture of platinum and palladium on an inert oxide support such as beta, eta, or gamma alumina. The process provides gasoline and distillate hydrogenation, but with limited hydrogenation activity. The process particularly avoids use of silica-alumina supports since use of silica-alumina in gasoline service can result in the conversion of high octane benzene into substantially lower octane cyclohexane.
We have surprisingly found that catalysts and processes having a catalyst incorporating metal mixtures of platinum and palladium onto a support comprising borosilicate, result in substantially improved hydrogenation compared to prior art hydrogenation processes including processes having a catalyst incorporating platinum and/or palladium on an alumina support or on supports comprising various zeolites. These results are more surprising and unpredictable in view of the fact that it has also been found that catalysts and processes having a catalyst incorporating metal mixtures of platinum and palladium onto a support comprising other 10-membered ring pentasils, such as ZSM-5, are less effective for hydrogenation.
We have also found that processes having a catalyst incorporating metallic mixtures of platinum and palladium, together incorporated onto a support comprising borosilicate, result in substantially improved hydrogenation compared to processes utilizing physical mixtures of catalysts comprising platinum incorporated onto a borosilicate support and palladium incorporated onto a borosilicate support. This particular synergy is more profound (and in contradistinction to the teachings of Rylander) since physical mixtures of platinum and palladium on a support comprising borosilicate have been shown not to provide improved hydrogenation.
It is therefore an object of the present invention to provide a process and catalyst that provide improved distillate aromatics saturation.
It is an object of the present invention to provide a process and catalyst that provide improved distillate desulfurization and denitrogenation.
It is an object of the present invention to provide a process and catalyst that increase distillate cetane number.
It is an object of the present invention to provide a process and catalyst that expand the volume of the distillate feedstock.
Other objects appear herein.